Implantable cardiac devices (“ICDs”) such as pacemakers, defibrillators or implantable cardiac monitoring devices may be used in the treatment of patients who have impaired cardiac function. For example, some ICDs may stimulate a patient's heart to maintain regular contractions of the heart thereby promoting blood circulation within the patient. Such stimulation may be prescribed when the patient's heart does not function normally due to, for example, a genetic or acquired condition.
In a healthy heart, contractions occur first in the muscles associated with the atrium chambers of the heart, followed by contractions in the muscles associated with the larger ventricle chambers of the heart. In this way, the atria assist in the filling of the ventricle chambers with blood returning from the veins. This enables the ventricles to more efficiently pump blood to the arteries.
Given the interaction of these chambers, efficient operation of the heart is predicated on each of the chambers operating in a proper timing sequence and having contractions that pump a sufficient amount of blood from the chamber. For example, during contraction the right atrium chamber should pump enough blood to effectively “fill” the right ventricle chamber. Moreover, this should occur immediately before the right ventricle begins to contract. In this way, the heart may efficiently pump blood on a repetitive basis.
A healthy heart repetitively contracts in the above described manner in response to the generation and conduction of electrical signals in the heart. These electrical signals are generated in and conducted through the heart during every beat of the heart.
Under certain circumstances, an ICD may compensate for abnormal operation of a heart by pacing (e.g., stimulating) one or more of the atria and/or ventricles. To stimulate the heart, a typical ICD generates a series of electrical signals which are applied to the heart via one or more electrodes implanted in the heart (e.g., in ventricular or atrial chambers). These electrical signals cause the heart to contract in much the same way as the native electrical signals discussed above cause the heart to contract.
To provide appropriate timing for the generation of electrical signals, conventional ICDs may sense signals in the heart. For example, an ICD may sense electrical signals in the atria to detect when the atria are being activated. The ICD may then delay a prescribed period of time after which it senses electrical signals in the ventricles to determine whether to apply a stimulus to the ventricles. In this way, the ICD may stimulate the ventricles at the appropriate time in an attempt to maintain efficient operation of the heart.
The signals from the sensors in the heart are collected in the ICD to be analyzed to determine when a stimulus should be generated or other action taken. The data received from the sensors is stored as a time series of data. The time series includes a set of time indicators and a paired set of data samples. The time indicators record a relative or absolute time that a corresponding data sample was taken by a sensor. Each data sample includes a value that measures a signal, such as an amplitude of the signal captured by the sensors, at the indicated time. This time series data is analyzed by, for example, morphology detection algorithms and/or arrhythmia detection algorithms to identify cardiac activity (e.g., P-waves corresponding to atrial activity, R-waves corresponding to ventricular activity, and their origin, etc.). Based on the timing or absence of cardiac activity, the ICD may determine the timing for the application of electrical stimulation pulses to the heart.
Storing the time series data may require significant storage space and analyzing the data may be computationally intensive. As a result, current ICDs may limit the resolution and quality of the analysis of the data. In practice only a small subset of data may be stored and analyzed due to the limitations of the battery power of the ICD on computation and the space limitations on the size of memory in the ICD.